Distorted thread locknut
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Template:Copy to Wiktionary An oval lock nut is a nut that has been deformed at the outside end so that it is no longer perfectly circular. As the nut is threaded on, the deformed end is forced back into a circle, holding tight to the bolt and preventing it from turning. Cleveloc, a popular brand of oval lock nut, is a registered trade name of Forest Fasteners.